Remove BASH_ENV from the build environment

This runs an arbitrary shell script for all non-interactive bash sessions, and we've got lots of those.

It was causing problems with Fedora 30, which triggers the `manpath` executable, which isn't available in our $PATH:
https://groups.google.com/d/msg/android-building/WqEyZQuxYtY/zDXl9irOCAAJ

Change-Id: If2f63bbbcc3accb8d239f93fa06823d301d69488
diff --git a/ui/build/config.go b/ui/build/config.go
index c8670ae..876bfe0 100644
--- a/ui/build/config.go
+++ b/ui/build/config.go
@@ -144,6 +144,7 @@
 		"DIST_DIR",
 
 		// Variables that have caused problems in the past
+		"BASH_ENV",
 		"CDPATH",
 		"DISPLAY",
 		"GREP_OPTIONS",